Cone pulley transmissions, in which two axially shiftable pairs of cone pulleys are coupled by an endless belt are well known; the spacing of the pulleys determines the radial position of the belt and, hence, the transmission ratio between shafts to which the pulley pairs are connected. The individual pulleys of the pairs can be axially positioned by hydraulic pressure systems. The hydraulic pressure arrangements may be connected to diagonally oppositely located pressure spaces, so that the wedge cones will be symmetrically positioned. Various other ways of placing the pulleys are known.
U.S. Pat. No. 4,229,988, RATTUNDE, shows a combined mechanical-hydraulic pressure arrangement. The mechanical pressure arrangement is positioned in parallel to the hydraulic pressure arrangement, that is, the cone disks have the sum of the mechanical pressure and hydraulic pressure applied thereto, so that the axial force is a combined hydraulically and mechanically generated pressure.
Completely hydraulically operated transmissions have the disadvantage that, upon failure of the hydraulic pressure, for example due to a leak in a hydraulic line, failure of the pressure pump, or the like, the transmission may become inoperative. Use of infinitely variable ratio transmissions in drive systems requiring the utmost in reliability, for example in automotive transmissions, therefore is impaired. The combined mechanical-hydraulic arrangement of the referenced patent permits generation of some mechanical pressure; it is, however, comparatively complex and hence costly in construction.